Transistors, for example, thin-film transistors (TFTs), may be made using inorganic or organic materials such as amorphous silicon, polycrystalline silicon, nano-crystalline silicon, zinc oxide (ZnO), InGaZnO, pBTTT polymers, etc. In many cases, the transistor may be subject to degradation over time, causing instability in the transistor's operation. Degradation and/or instability of a transistor could be caused by various factors, such as electrical stress, light exposure, mechanical strain/stress, environment temperature and moisture etc. In particular, degradation of a transistor can cause instability in the provision of current to a load that is connected to the transistor.
For example, in light emitting displays, such as light emitting diode (LED) displays or organic light emitting diode (OLED) displays, degradation of the transistor driving a light-emitting device may result in inconsistent light-emitting device drive current, and, as a result, inconsistent brightness of the light-emitting device. The resulting degradation of the brightness of the light-emitting device may reduce the lifetime of the display and cause visual non-uniformities in the display.
Electrical instability of a transistor may be characterized as current fluctuation and/or a threshold voltage shift (Vt-shift, ΔVT). A conventional simple voltage programmed two-transistor pixel circuit may not fully compensate for light emitting device current instability caused by ΔVT of the drive transistor due to electrical stress. Therefore, it is desirable to compensate ΔVT so as to stabilize the drive current provided by the drive transistor to the load.
In a display, voltage compensation can be used to compensate for the degradation of the drive transistor to minimize the instability of the drive current provided from the drive transistor to the light-emitting device. It will be understood that voltage compensation may also be useful in steady state lighting and other situations where a stable drive current is needed.
There are known methods and circuits for compensating for threshold voltage shift of a drive transistor. However, these conventional methods have limitations.
In displays, conventional ΔVT-compensation methods include current programming methods and voltage programming methods. The current programming methods typically use two transistors in series with an electroluminescent device, causing higher static power consumption. The higher power consumption may be undesirable in some applications of displays, such as portable electronics, where power consumption is critical to battery life. The relatively slow programming speed of conventional programming methods can limit the size of the display and programming speed may be particularly slow for smaller programming currents and/or larger display sizes.
On the other hand, voltage programming methods typically use specialized cycles during a programming phase to compensate for electrical instability but require longer programming times, complicated control signals, and complicated external drivers. Limited ΔVT-generation speed results in a lower programming speed. Since more than one transistor is typically used in the current path of the light-emitting device, higher power is consumed. The increased power consumption may be undesirable in low-power applications such as AMOLED displays in portable electronics. Control signals may be complicated and increase the complexity of the external driver.
Conventional circuits for ΔVT-compensation can also include optical feedback provided by a photo-sensor to correct the decay of OLED luminance. The photo-sensor may complicate the pixel circuit and take up pixel area, resulting in lower aperture ratio and resolution. Instability of the photo-sensor and light interference from the environment and neighbouring pixels may also cause errors in the feedback loop.
Conventional pixel circuits for ΔVT compensation may also use an external driver (e.g. a complementary metaloxidesemiconductor (CMOS) driver) to detect and correct the ΔVT of the drive transistor. External drivers may be intended to compensate for ΔVT but these approaches are complicated. Methods using external drivers to detect and compensate for ΔVT of the drive transistors generally have limited compensation resolution. The number of pixels which can be monitored by the external driver is limited by the pixel measurement speed, so the resolution of ΔVT-compensation is limited.
It is, therefore, desirable to provide an improved apparatus and method for electrical stability compensation.